The present invention relates to a binder fiber and a nonwoven fabric using the fiber. Nonwoven fabrics using such a fiber are suitable for use especially as cushion material because they are so soft in hand that, even after prolonged use or when used in a high temperature atomosphere, they are unlikely to flatten and not liable to any appreciable decrease in their adhesion strength.
Hitherto, various known types of nonwoven fabrics in which principal fibers are bonded with binder fibers have been used in applications such as filters, interlinings, shoulder paddings, furniture stuffings such as sofa cushion, chair back cushion, and other cushion paddings, and cushion material for beds and automotive seats.
A binder fiber of the type which has hitherto been largely used has its binder component comprised of polyester copolymer including ethylene terephthalate and ethylene isophthalate. This polyester has high rigidity and is a non-crystalline polymer which does not exhibit any definite crystalline melting point but begins to soften at temperatures above the glass transition point (about 65.degree. to 70.degree. C.). Known nonwoven fabrics which are manufactured by subjecting a combination of the principal fiber and the binder fiber to the process of heat bonding have a disadvantage that they lack handle flexibility and feel rather hard. Another disadvantage is that when subjected to repeated compression and/or bending, the nonwoven fabric is liable to joined-spot fracture, resulting in becoming flattened, or that when used in a high temperature atmosphere, the nonwoven fabric is subject to bond deterioration, resulting in deformation of the fabric.
It is also known to use polyurethane foam material largely in applications including furniture stuffings, such as seat and back cushions for sofas and chairs, and cushionings for beds and automotive seats. With polyurethane foams, however, problems have been raised from the standpoints of safety and environmental protection that they produce nitrogen-containing toxic gases when combusted, and that production of such a foam material requires the use of a fluorocarbon gas which may lead to depletion of ozone layer above the atmosphere.
Then, as a material which can replace polyurethane foam, it is conceivable to use a nonwoven fabric formed principally of a polyester fiber. In this regard, several types of nonwoven fabrics have been known including one formed by needling polyester fiber webs; one using polyester fiber and binder fiber components in combination which are fusion bonded into the nonwoven fabric form (as described in, for example, Japanese Patent Application Laid-Open No. 57- 35047); and one using polyester elastomer as a binder component as in the case of aforesaid nonwoven fabric (as described in, for example, Japanese Patent Application Laid-Open No. 4-240219).
Unfortunately, such known polyester nonwoven fabrics also have their own drawbacks. The one made by needling polyester fiber webs is disadvantageous in that some component fibers are likely to fall off or fly away. The one which is intended to overcome this drawback by incorporating binder fibers through heat bonding lacks softness and feels rather hard. Both of these types are likely to flatten due to repetitive compression or under compression in a high temperature atmosphere, and are also liable to deterioration with time in their cushioning properties while in use.
The one incorporating aforementioned polyester elastomer as the binder component is intended to eliminate the shortcomings of known binder fibers. However, the polyester elastomer disclosed in Japanese Patent Application Laid-Open No. 4-240219 is one produced by copolymerization with a poly(alkylene oxide) glycol component so that it is rather liable to heat degradation and is less heat bondable.